An electron emission element for emitting electrons by utilizing field electron emission has been conventionally known. The field electron emission is a method for emitting electrons, wherein electrons are emitted by applying a voltage across two electrodes, and the application of the voltage forms a high electric field between the electrodes, thereby causing electrons to be emitted from one (emitter) of the electrodes due to a tunnel effect. Different types of field electron emission elements are known, such as a spint-type field electron emission element and a carbon nanotube (CNT) field electron emission element, which are different in emitter structure.
There has been a demand for an electron emission element capable of being used in the air. It is, however, theoretically difficult to operate, in the air, the electron emission element which carries out the field electron emission. The reason for such difficulty in operation is as follows: a high electric field is required for the field electron emission, and therefore, emitted electrons have a high energy. Collision of such high-energy electrons with gas molecules in the air ionizes the gas molecules. Cations generated by the ionization are (i) accelerated toward a surface of the electron emission element by the high electric field which is formed in the vicinity of the electron emission element and (ii) collided with the surface. This ultimately causes sputtering. The sputtering destroys the electron emission element. It is further known that collision of the high-energy electrons with oxygen molecules does not ionize the oxygen molecules but generates ozone. Ozone is a remarkably active and toxic material, and deteriorates various materials.
On this account, the electron emission element which carries out the field electron emission is generally used in a sealed vacuum. In a case where it is necessary to take electrons out from the sealed vacuum to the air, the electrons should be emitted into the air via an electron transmission window which separates a vacuum layer from the air.
On the other hand, electron emission elements called an MIM type electron emission element and an MIS type electron emission element are known each as an electron emission element capable of stably operating in the air (see Patent Literature 1). The MIM type electron emission element is made up of three layers of a thin film metal electrode, an insulating layer, and a metal electrode substrate. The MIS type electron emission element is made up of three layers of a thin film metal electrode, an insulating layer, and a semiconducting electrode substrate. The insulating layers are hereinafter referred to as electron accelerating layers because electrons are accelerated in the insulating layers of the respective MIM type and MIS type electron emission elements. By applying a voltage across electrodes which sandwich each of the electron accelerating layers, electrons are accelerated in each of the electron accelerating layers, and some of the electrons are (i) caused to transmit the thin film metal electrodes of the respective MIM type and MIS type electron emission elements and (ii) two-dimensionally emitted from whole surfaces of the respective MIM type and MIS type electron emission elements.
The MIM type and the MIS type electron emission elements are identical to the electron emission element which carries out the field electron emission, in that electrons are (i) accelerated by an electric field formed between electrodes and (ii) emitted outside of an electron emission element. However, the MIM type and the MIS type electron emission elements are greatly different from the electron emission element which carries out the field electron emission, in that in a case of the MIM type and the MIS type electron emission elements, formation of an electric field during operation of an electron emission element is limited to within the electron emission element. Such limitation allows the MIM type and the MIS type electron emission elements to stably operate in the air.
The MIM type and the MIS type electron emission elements are (i) thus capable of stably operating in the air and (ii) surface emission type electron emission elements.